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My understanding is that hormones generated by the thyroid gland, including, for example, T4, are excreted and recirculated in the body through the digestive tract. The reason for thinking this is that bile acid sequestrants such as cholestyramine cause a lowering of serum T4. In fact, patients who have hypothyroidism are explicitly warned not to use sequestrants for this exact reason. Here is a statement from a clinical trial discussing the effect:

The enterohepatic circulation of thyroid hormones is increased in thyrotoxicosis.Bile-salt sequestrants (ionic exchange resins) bind thyroid hormones in the intestine and thereby increase their fecal excretion. Based on these observations, the use of cholestyramine has been tried. The present study evaluates the effect of low doses of cholestyramine as an adjunctive therapy in the management of hyperthyroidism. -- clinical trial NCT00677469

Therefore, there would appear to be a metabolic pathway in which T4 is being excreted into the digestive tract and then is later re-absorbed in the lower intestine somehow.

What is this metabolic pathway? How exactly does this mechanism work? Does it have a name?

For more information on thyroid hormone sequestration see:

https://clinicaltrials.gov/ct2/show/NCT00677469

http://www.ncbi.nlm.nih.gov/pubmed/8435884

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  • $\begingroup$ can you name a primary article that demonstrates that bile acid sequestrates lower circulating T4? Also, I'm not following why you state that T4 is excreted into the digestive tract (?) - can you clarify? T4 is a plasma hormone that acts on nuclear receptors in a wide array of tissues/organs and I've never heard that T4 is secreted into the GI tract and then reabsorbed... bile acids on the other hand are secreted into the GI tract and actively reabsorbed in the distal small intestine $\endgroup$ – Vance L Albaugh May 14 '16 at 18:47
  • $\begingroup$ @VanceLAlbaugh I have updated my question with further details. $\endgroup$ – Imprisoned Rhesus May 14 '16 at 18:59
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I have partially answered this question by discovering that the metabolic pathway is called enterohepatic circulation. In this interaction bile salts are absorbed and transported out of ileal mucosal cells by binding to albumin.

However, it is still unkown to me exactly how it is that thyroxine and other hormones are moderated in this process and how it works. Does the albumin collect thyroxine as well as bile salts? How is the process moderated? In other words, bile salts are allowed to be excreted according to some moderated control mechanism. What is this mechanism?

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  • $\begingroup$ Since you're answering your question, googling "Clearing of Metabolic Waste via Enterohepatic Recirculation" might help. Actually, googling enterohepatic recirculation and anything to do with thyroid hormone(s) will help. $\endgroup$ – anongoodnurse May 14 '16 at 21:52
  • $\begingroup$ This is an interesting question, but, unfortunately, there is little exact information available. In 1990, de Herder wrote a dissertation on the topic, which is available from repub.eur.nl/pub/50862/900411_HERDER-Wouter-Willem-de.pdf . A second mechanism, by which bile acids act on metabolism of thyroid hormones is an endocrine effect, i.e. stimulation of type 2 deiodinase via the TGR5 receptor, see dx.doi.org/10.1038/nature04330 for details. $\endgroup$ – jwdietrich Jul 6 '16 at 20:30

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